What is a Meteorite?

A
meteorite
is a rock that was formed elsewhere in the Solar System, was orbiting the sun
or a planet for a long time, was eventually captured by Earth's gravitational
field, and fell to Earth as a solid object. A meteoroid is what we call the
rock while it is in orbit and before it is decelerated by the Earth's
atmosphere. A meteor
is the visible streak of light that occurs as the rock passes through the
atmosphere and exterior of the rock is heated to incandescence. Most (~99.8%)
meteorites are pieces of asteroids. A few rare meteorites come from the Moon
(0.1%) and Mars (0.1%).

What is a Lunar Meteorite?

Lunar
meteorites, or lunaites, are meteorites from the Moon. In other words, they
are rocks found on Earth that were ejected from the Moon by the impact of
an asteroidal meteoroid or possibly a comet.

How Did Lunar Meteorites Get Here?

Meteoroids
strike the Moon every day. Lunar escape velocity averages 2.38 km/s (1.48 miles
per second), only a few times the muzzle velocity of a rifle (0.7-1.0
km/s). Any rock on the lunar surface that is accelerated by the impact of a
meteoroid to lunar escape velocity or greater will leave the Moon's gravitational
influence. Most rocks ejected from the Moon become captured by the
gravitational field of either the Earth or the Sun and go into orbit around
these bodies. Over a period of a few years to tens of thousands of years,
those orbiting the Earth eventually fall to Earth. Those in orbit around
the Sun may also eventually strike the Earth up to a few tens of millions
of years after they were launched from the Moon.

Words
That Confuse People

·meteor
– The visible light that occurs when a meteoroid passes through the
Earth's atmosphere

·meteorite
– A rock found on Earth that was once a meteoroid.

These
are simple definitions. A more technical but accurate definition of a
meteorite is given by Alan E. Rubin and Jeffrey N. Grossman (2010):

"A meteorite is a natural, solid object larger than 10 µm in size,
derived from a celestial body, that was transported by natural means from
the body on which it formed to a region outside the dominant
gravitational influence of that body and that later collided with a
natural or artificial body larger than itself (even if it was the same
body from which it was launched)."

A road sign in Newfoundland

How Do We Know That They Are Meteorites?

On
a broken or sawn face, all lunar meteorites look like some kinds of Earth
rocks, even to an experienced lunar scientist. We can often tell that they
came from space, however, because many lunar meteorites have fusion crusts (the olive-green crust on the
photo above) from the melting of the exterior that occurs during their passage
through Earth's atmosphere. On meteorites found in hot deserts, the fusion
crusts sometimes have weathered away. However, as explained in more detail
below, all meteorites contain certain isotopes (nuclides) that can only be
produced by reactions with penetrating cosmic rays while outside the
Earth's atmosphere. The presence of "cosmogenic nuclides" is the
ultimate test of whether or not a rock is a meteorite. All lunar
meteorites that have been tested show evidence of cosmic-ray exposure.

How Do We Know That They Come From the Moon?

Chemical
compositions, isotope ratios, mineralogy, and textures of the lunar meteorites
are all similar to those of samples collected on the Moon during the Apollo
missions. Taken together, these various characteristics are different from
those of any type of terrestrial rock or other type of meteorite. For
example, all of those meteorites in the List that are classified as feldspathic breccias
are rich in the mineral anorthite,
which is a plagioclase feldspar, mineralogically, and a calcium aluminum
silicate, chemically. Consequently, these meteorites all have high concentrations
of aluminum and calcium. Because of some unique aspects about how the Moon
formed, the lunar highlands are composed predominantly of anorthite.
Anorthite is much less common on asteroids and, to the best of our
knowledge, on the surface of any other planet or planetary satellite.

How Many Are There?

It
depends upon how one counts. More than 240 named stones have been described
in the scientific literature that are lunar meteorites. Other rocks that
have not yet been described in the scientific literature but which might be
lunar meteorites are being sold by reputable dealers. The complication is
that some to many of these stones are "paired," that is, two or
more of the stones are different fragments of a single meteoroid that made
the Moon-Earth trip. When confirmed or strongly suspected cases of pairing
are taken into account, the number of actual meteoroids reduces to about 118.
Pairing has not yet been established or rejected for the many recently
found meteorites, so the actual number is not known with certainty. In the List, known or strongly suspected
paired stones are listed on a single line separated by slashes. In most
cases, the stones were found close together because a meteoroid broke apart
upon encountering the Earth's atmosphere, hitting the ground or ice, or
while traveling within the ice in Antarctica. (In the other cases, all from
northern Africa, we don't know for sure where they were found.) The six LaPaz Icefield stones all have fusion crusts
and the broken edges don't fit together, thus the LAP meteoroid likely
broke up in the atmosphere. Among the numerous Dhofar lunar meteorite
stones, about 15 appear to all be
pieces of a single meteorite.

Pairing and
Naming

Although
it is often confusing, meteorite scientists refer to all found pieces of
a meteoroid as a single meteorite, ideally with a single name. Thus, Allende
refers to hundreds of fragments of a single 2-ton meteoroid that broke
apart over Mexico in 1969. All the pieces are paired stones of a fall and
they are all called Allende.
With finds (meteorites not observed to fall) different stones are often
given different names because they are found at different times. If later
studies show two stones to be paired, then one of the names is officially
discarded. With the Antarctic and hot-desert meteorites, however, all the
stones are originally given different designations because so many
meteorites are found in a small area. This problem leads to the awkward
combination names like Yamato
82192/82193/86032 when one is referring to "the meteorite,"
in the accepted sense, as opposed to the individual stones. If the 15
stones of the Dhofar 303 et al. lunar
meteorite had been found, for example, in the U.S., they would likely all
been given the same name.

Do All Lunar Meteorites Come from One Big Impact on the
Moon?

The
lunar crater Daedalus, about 93 kilometers (58
miles) in diameter, was photographed by the crew of Apollo 11 as they
circled the Moon in 1969. NASA photo AS11-44-6611.

For
several reasons, we know that the lunar meteorites derive from many
different impacts on the Moon. The textural and compositional variety
spans, and in some ways exceeds, that of rocks collected on the six Apollo
landing missions, so the meteorites must come from many locations. More
importantly, it is possible to determine how long ago a rock left the Moon
using cosmic-ray
exposure ages. Small rocks on the surface of the Moon and in
orbit around the Sun or Earth are exposed to cosmic rays. The cosmic rays
are so energetic that they cause nuclear reactions in the meteoroids that
change one nuclide (isotope) into another. Some of those nuclides produced
are radioactive. As soon as they fall to Earth, production stops because
the Earth's atmosphere absorbs nearly all cosmic rays. The radionuclides
decay on Earth with no further production. The most well-know such isotope
is 14C (carbon 14), which is produced from oxygen atoms in the
meteoroid. Other important radionuclides produced by cosmic-ray exposure
are 10Be, 26Al, 36Cl, and 41Ca.
Because the various radionuclides all have different half-lives, it is
often possible to tell how long a rock was exposed on or near the surface
of the Moon, how long it took to travel to Earth, and how long ago it fell.
For example, cosmic-ray exposure data for Kalahari
008/009 suggest that the meteorite left the Moon only a few hundred
years ago. At the other extreme, Dhofar
025 took 13-20 million years to get here (Nishiizumi & Caffee, 2001). Because
there is a wide range in the Earth-Moon transit times, we know that many
impacts on the Moon were required to launch all the lunar meteorites.

Does It Take a Big Impact to Launch a Lunar Meteoroid?

Vogt
et al. (1991) estimated that the frequency of impacts on the
Moon large enough to eject lunar meteorites is greater than 5 per million
years. On the basis of impact probability and the known size distribution
of lunar craters, Paul Warren (1994) makes a persuasive case that lunar meteorites
come from relatively small craters — those of only a few kilometers in
diameter. The main thrust of his argument is that all the lunar meteorites
were blasted off the Moon in the last ~20 million years (most in the last
few hundred thousand years) and that there haven't been enough
"big" impacts on the Moon in that time to account for all the
different lunar meteorites. As new lunar meteorites are found each year,
Warren's argument becomes more valid. James Head (2001) calculates on a theoretical basis that impacts
causing craters as small as 450 m (about a quarter of a mile) in diameter
can launch lunar meteorites. More recently, Basilevsky et al. (2010)
argue on the basis of the known number of lunar meteorites and the frequency
of impacts on the Moon that "a significant part of the lunar meteorite
source craters are not larger than a few hundreds of meters in
diameter." (That's big if it happens in your backyard, but it's not so
big for the whole Moon.) If lunar meteorites come from such small craters,
it would be especially difficult to locate the actual source crater of a
particular lunar meteorite.

Prediction
19 Years Before the First Lunar Meteorite Was Recognized

"The
occurrence of secondary craters in the rays extending as much as 500 km
from some large craters on the moon shows that fragments of considerable
size are ejected at speeds nearly half the escape velocity from the moon
(2.4 km/sec). At least a small amount of material from the lunar surface
and perhaps as much or more than the impacting mass is probably ejected
at speeds exceeding the escape velocity by impacting objects moving in
asteroidal orbits. Some small part of this material may follow direct
trajectories to the earth, some will go into orbit around the earth, and
the rest will go into independent orbit around the sun. Much of it is
probably ultimately swept up by earth."
...
"There is also a possibility that fragments can be ejected at escape
velocity from Mars by asteroidal impact, though not as large a fraction
as is ejected from the moon. If some small amount of material escapes
from Mars from time to time, it seems likely that at least some small
fraction of this material would ultimately collide with earth."

Where on the Moon Did They Come From? Are Any from the Far
Side of the Moon?

Although
scientists like to speculate that a certain lunar meteorite came from a
certain crater or region of the Moon, no one has actually identified with
certainty the source crater from which any of the lunar meteorites
originated.

Schematic map of lunar impact basins on the nearside
and farside of the Moon. (Based on Figure 2.3 of The Lunar
Sourcebook.)

There
is some evidence and model results indicating that asteroidal meteoroids
strike the western (leading) hemisphere of the Moon (that is, the
"side" with Mare Orientale, which means east because astronomical
telescopes see the Moon upside down!) a bit more frequently than the eastern
hemisphere (the Mare Marginis "side"). On the other hand, lunar
meteoroids leaving the eastern hemisphere may have a slightly better chance
of reaching Earth. Overall, however, there's probably little East-West bias
in our lunar meteorite collection. There are reasons to expect that
asteroidal meteoroids strike the equatorial areas of the Moon a bit (1.28
times) more frequently that the polar regions.

There
are no reasons to suspect that lunar meteorites come from the nearside of the
Moon preferentially to the farside, or vice versa. So, half of the lunar
meteorites come from the far side of the Moon. It’s that simple. We just
don't know which ones those are. There is no scientific basis for a statement
in an advertisement on e-Bay:
"The ONLY LUNAR meteorite from the dark side of the moon." (Also,
of course, the "dark side" of the Moon keeps changing with lunar
phase! Except for some locations at the poles, any place in the dark will be
sunlit 14 days later.)

For
any given lunar meteorite, the probability is not exactly 50-50 that it came
from either the near side or the far side. There is more mare basalt on the
near side than the far side (FeO map below), so the chance is better than
50-50 that an iron-rich meteorite (mare basalt or basaltic breccia) is from
the near side and that an iron-poor meteorite (feldspathic) is from the far
side. As explained below, Sayh al Uhaymir 169,
Dhofar 1442, Northwest
Africa 4472/4485 and Northwest Africa 6687
must derive from the near side.

How Big Are They?

The
largest single stones are Kalahari 009
at 13.5 kg (30 lbs), Northwest Africa 5000
at 11.5 kg (25 lbs), and Shisr 162 at 5.5
kg (12 lbs). The largest named stone, which was found in several pieces, is NWA 10309 at 16.5 kg (36.4 lbs). At the other
extreme, several of the lunar meteorite fragments found in Antarctica and
Oman only weigh a few grams (a U.S. nickel weighs 5 grams). The smallest
named stones are Graves Nunataks 06157 at
0.788 g and Dar al Gani 1048 at 0.801 grams.

The plot
to the left shows the distribution of lunar meteorite masses (all stones of
a given meteorite). Masses in the 128-256 gram range are most common. The
plot on the right shows relative masses by continent or country. Botswana
is represented by a single, huge meteorite, Kalahari 008/009. Based on data
through February, 2016.

How Rare Are They?

Meteorites
are very rare rocks; lunar meteorites are exceedingly rare. It difficult to
assess how rare they really are. Of the ~41,100 meteorite stones found in
Antarctica, where record keeping has been superb, (1976-2014), 1 in 1200
meteorite stones is lunar (35 stones representing ~22 meteorites).

Another
measure of rarity is mass. The total mass of all known lunar meteorites is about
137 kg (303 lbs.). By comparison, the Allende
and Jilin
meteorites (both stony) are 2 and 4 metric tons (2000 and 4000 kg) each
while several iron meteorites weigh more than 10 tons! (e.g., Hoba, Gibeon,
Campo
del Cielo).

The
mass of all known lunar meteorites is now about 36% of the mass of the
Apollo lunar sample collection.

Lunar
Meteorites for Sale

Meteorites,
including lunar and martian meteorites, are easily available for purchase.
Samples (end cuts, slices, chips, crumbs, dust) of the lunar meteorites
sell on the Internet (e.g., e-Bay)
for between about $600 and $4,000 per gram, depending upon rarity
(perceived or real!) and demand. By comparison, the price of
24-carat gold is about $60 per gram and gem-quality diamonds start at
$1000-2000/gram. Prices have declined as the number of meteorites has increased.

Most rocks advertised on the Internet as lunar meteorites are, in fact, meteorites
from the Moon sold by reputable dealers. Some are not, however. Also, on
more than one occasion, I have seen samples advertised on e-Bay as one
particular lunar meteorite (e.g., Dhofar
081) when the sample in the photo is clearly from a different lunar
meteorite (e.g., Dhofar 911). Caveat emptor.

I've been contacted by seven men who have wanted to buy a lunar meteorite
to mount in a piece of jewelry for their girl friend, fiancée, or wife.
Be aware that compared to many gem stones, lunar meteorites are not
"hard" rocks and most have fractures from meteorite impacts on
the Moon. And although I love lunar meteorites, they are not all that
attractive compared to most gem stones. Get her a diamond, emerald, opal,
or agate!

Where, How, and When Are They Found?

In
the lingo of meteoritics, all lunar meteorites have been "finds;"
none are "falls." In other words, no lunar meteorite has been
observed as a meteor. This is a curious fact as there are fewer martian
meteorites than lunar meteorites yet several of the martian meteorites
were observed to fall (Chassigny, Shergotty,
Nakhla,
Tissint, Zagami).
No lunar meteorite has yet been found in North America, South America,
or Europe. We can reasonably assume that lunar meteorites have fallen
on these continents in the past 100,000 years, but if someone has found
one, it's not yet been recognized as a lunar meteorite.

Nearly
all lunar meteorites have been found in areas that are well known to be
good places to find meteorites. All such places are dry deserts where there
are geologic mechanisms for concentrating meteorites, where rocks of
terrestrial origin are rare, or where meteorites do not weather away
quickly from exposure to water.

Many
lunar meteorites have been found in Antarctica (see "Why Antarctica")
by expeditions funded by the U.S. (ANSMET)
or Japanese (NIPR)
governments. Most of lunar meteorites have been found in the Sahara Desert
of northern Africa and in the desert of Oman - all since 1997. Meteorites
from hot deserts are almost exclusively found by local people or experienced
collectors.

Allan
Hills 81005 (ALHA 81005), the first meteorite to be recognized as
originating from the Moon, was found during the 1981-82 ANSMET collection
season, on January18, 1982. The three Yamato
79xxx meteorites were collected earlier, but not recognized to be of
lunar origin until after 1982. The first lunar meteorite to be found
appears to be Yamato 791197, on 20 November 1979. However, it is not known
when Calcalong
Creek was found. The Meteoritical
Bulletin says "after 1960,"
but it was not recognized to be of lunar origin until 1990, so it may well
have been collected earlier than Yamato 791197.

Shişr 166 was found at night with a
flashlight. Oued Awlitis 001 was found
embedded in roots of a dead tree during a search for firewood.

ANSMET 1988-89 field team searching
for meteorites in "Meteorite Moraine" near Lewis Cliff. Photo
by Robbie Score.

Searching for meteorites in Morocco.
Photo courtesy of HasnaaChennaoui.

The first lunar meteorites were found
in Antarctica in 1979. In 1997 the first lunar meteorite was found in the
Sahara Desert and since 1999 many have been found in Oman (Arabian
Peninsula). Data through February 2016.

How Do I Recognize a Lunar Meteorite?

Although
the discovery that there are rocks on Earth that originated from the Moon is
relatively new, lunar rocks have surely been dropping from the sky throughout
geologic history. Mikhail Nazarov and colleagues of the Vernadsky Institute
in Moscow estimate that "several tens or few hundred kilograms" of
lunar rocks in the mass range of 10-1000 g strike the Earth's surface every
year. That fact does not make lunar meteorites easy to find or recognize,
however. Under ideal conditions (e.g., Antarctica), some lunar meteorites are
almost instantly recognizable as lunaites because they have fusion crusts that are highly vesicular. No Earth rock and no other kind of
meteorite has a crust that is as vesicular as that of lunar meteorites QUE 93069 or PCA
02007. Some lunar meteorites (the basalts) do not have such vesicular
fusion crusts, however, and the fusion crust of most lunar meteorites found
in hot deserts has been ablated away by the wind. In the absence of a fusion
crust, a lunar (or martian) meteorite is less likely to be recognized as a
meteorite than is an asteroidal meteorite because it more closely resembles
terrestrial rocks in mineralogy and density. A
weathered lunar meteorite would not be an impressive or suspicious looking
rock if found in a cornfield or streambed (see Dar
al Gani 400 or QUE94281) and a brecciated lunar meteorite could easily be
overlooked in the field as a terrestrial sedimentary or volcaniclastic
rock. Even experienced meteorite collectors admit that Kalahari 009 does not "look like"
any kind of meteorite. Lunar meteorites contain a much smaller amount of
metal than ordinary chondrites, so most are not attracted to a magnet. Also, they have densities similar to
terrestrial rocks; they're not heavy for their
size, as are most meteorites. Although I had been studying Apollo lunar
rocks for 18 years, I did not recognize the MAC88105
lunar meteorite as a Moon rock when another member of the 1988 ANSMET team
handed it to me in the field and asked "What do you think about this
one?" Unfortunately, lunar meteorites and some kinds of Earth rocks
strongly resemble each other in hand specimen. Bottom line: Even for an
expert it's not usually possible to identify a lunar meteorite just "by
looking." Only expensive and time-consuming tests can prove that a rock
is a lunar (or martian) meteorite. "Looks like" is not a good test
for lunar meteorites. People have sent me photos of broken concrete that they
claim "looks like" some of the photos of lunar meteorites on my
website.

How Are They Named?

By
long-standing convention, meteorites are named after the location where
they fall or are found. For example, Calcalong
Creek is a place in Australia. Somewhat contrary to the convention, the
Antarctic meteorites in the U.S. collection often go by abbreviated names,
where ALHA = Allan Hills, EET = Elephant Moraine, GRA = Graves Nunataks, LAP
= LaPaz Icefield, LAR = Larkman Nunatak, MAC = MacAlpine Hills, MET =
Meteorite Hills, MIL = Miller Range, PCA = Pecora Escarpment, and QUE =
Queen Alexandra Range. Similarly, the Dar al Gani (Libya), Northeast
Africa, Northwest Africa, and Sayh al Uhaymir meteorites are sometimes
abbreviated DaG, NEA, NWA, and SaU. Because hundreds to thousands of
meteorites have been found in Antarctica and hot deserts, serial numbers
are used in addition to names. For the Antarctic meteorites, the first two
digits of the numeric part of the name represents the collection year. (See
map of Antarctic meteorite locations for
the U.S. collection.)

What's the Difference Between a Lunar Meteorite and a Tektite?

A
lunar meteorite is a rock from the Moon. A tektite is not a meteorite
(it never orbited the sun or Earth) and it is not from the Moon. A tektite
was formed from Earth material during the impact of a meteoroid.

Tektites
consist of glass and are often shaped like spheres, dumbbells, or teardrops.
Lunar meteorites never have such interesting shapes and none are composed
entirely of glass. Tektites have compositions
like terrestrial rocks, not like lunar rocks.

How Are Lunar Meteorites Classified?

Lunar
rocks are classified by the minerals they contain (mineralogy), how the
mineral grains are put together (texture), how the rock formed (petrology),
and chemical composition (chemistry). These different parameters sometimes
leads to confusion because a geochemist might call a rock
"feldspathic" (dominant mineral) or "aluminum rich"
(chemical composition) while a petrologist might call it an
"anorthosite" (mineral proportions and implied mode of formation)
or "regolith breccia" (texture
and and type of rock components).

Since
the time of Galileo, the lunar surface has been divided into two types of
terrane, the mare
(pronounced mar'-ay, which is the Latin word for sea) and the terra
(land) or highlands.

Feldspars are some of the
most common minerals of the crust of the Earth and Moon. Rocks of the lunar
highlands contain a high proportion (60-99%) of a type of feldspar known as plagioclase.
In particular, the plagioclase of the lunar highlands is the calcium-rich
variety known as anorthite
(the more sodium-rich varieties are rare on the Moon). Mineralogically, a
rock composed mostly of the anorthite is called an anorthosite,
and most rocks of the lunar highlands are, in fact, anorthosites. Lunar
scientists often refer to the highlands crust as "feldspathic,"
indicating the major mineral, or "anorthositic," indicating the
major rock type. Anorthite, like all forms of feldspar, is rich in aluminum
and poor iron.

Rocks
from the maria are classified as basalts
because they are crystalline, igneous rocks (texture) consisting mainly of
pyroxene and plagioclase (mineralogy). Specifically, they are called mare basalts
because they formed when magmas from inside the Moon erupted (petrology) into
the basins formed by the impacts of small asteroids early in lunar history to
form the maria. Mare basalts are subclassified by
chemical composition (chemistry), for example, "low-titanium (Ti) mare
basalt." Mare basalts are rich in iron
because they contain pyroxene, olivine, and ilmenite, all of which are
iron-rich minerals, and the amount of pyroxene + olivine + ilmenite exceeds
the amount of iron-poor plagioclase.

NWA
2995 is a fragmental breccia (2.5-mm grid in background). Note
that in this and other brecciated lunar meteorites, the clasts are not
particularly colorful. The "gray-scale" nature of brecciated lunar
meteorites distinguishes them from many terrestrial sedimentary rocks
(e.g., meteorwrong no. 124)
which are reddish because they contain ferric iron (hematite).Some lunar meteorites
from hot deserts are more colorful than lunar meteorites from Antarctica
because the hot-desert meteorites have suffered a greater degree of
chemical alteration from interaction with liquids since landing on Earth.
Many lunar meteorites from Oman (e.g., Dhofar
303 and paired stones) are pinkish as a result of terrestrial
alteration (hematite staining).

Breccias

Breccias are rocks made up
of bits and pieces of other rocks (clasts) in a matrix of finer-grained
rock fragments, glass, or crystallized melt.

Monomict
breccia is a term applied to a breccia that is made up entirely
one kind of rock. Monomict breccias are rare on the Moon because
meteoroid impacts tend to mix different kinds of rocks.

Dimict
breccias or dilithologic
breccias are made up of only two lithologies. The term is
usually applied to a common type of rock collected on the Apollo 16
mission that consists of anorthosite (light color) and mafic (dark, iron
rich) crystallized impact melt in a mutually intrusive textural
relationship. SaU 169, however, could be
regarded as a dilithologic breccia.

Polymict
breccia is a general term that encompasses all breccias that
aren't either monomict or dimict. Types of polymict breccias are glassy
melt breccias, impact-melt breccias, granulitic breccias, regolith breccias, and fragmental
breccias. Each of these breccia types has a different texture because the
set of conditions that formed them differed.

An impact-melt
breccia can be regarded as in igneous rock because it formed
from the cooling of a melt. Regolith
and fragmental breccias are the closest lunar equivalents to
terrestrial sedimentary rocks. Granulitic
breccias are metamorphic rocks in that they were some other
type of breccia that was metamorphosed (recrystallized) by the heat of an
impact.

Igneous
anorthosites are rare in the lunar highlands, but some were found on the Apollo
missions. Impacts of asteroidal meteorites on the Moon both break rocks of
the lunar crust apart and glue them back together. All lunar meteorites from
the highlands are breccias
(pronounced brech'-chee-uz), a textural term
for a rock that is composed of fragments of other rocks and that is held together
by shock compaction or by material that was partially or totally molten. An
impact can melt rock, forming impact
melt. The melt usually collects rock fragments called clasts as
it is forced away from the point of impact within a crater. When the melt
cools, it forms an impact-melt
breccia - clasts suspended in a matrix of solidified (glass or
crystalline) impact melt.

The
lunar surface is covered with fine-grained material called soil or regolith.
The shock wave associated with an impact can lithify the regolith - it can
turn the fine, powdery material into a coherent rock called a regolith
breccia. At depth, coarser fragments can be lithified to
form a fragmental
breccia. Breccia
is a textural term that applies to rocks of both the maria and highlands.
Most lunar meteorites are feldspathic
regolith breccias, that is, rocks consisting of lithified soil
from the lunar highlands. Most highlands rocks are breccias because
the highlands crust is very old and the impact rate was greater in early
lunar history than during the time since the magmas forming mare basalts
erupted.

The lunar crust is formed mainly from a
light-colored, aluminum-rich mineral known as anorthite, a plagioclase feldspar.
Early in lunar history the crust was impacted by small asteroids to form
large craters called basins. Dark, iron-rich magmas generated from melting
inside the Moon erupted into the basins. To ancient astronomers the
resulting dark, circular features resembled seas. They were given Latin
names like Mare Serenitatis, the "Sea of Serenity."

Rocks from the lunar highlands are rich
in aluminum and poor in iron because they are composed mainly of feldspar.
Rocks from the maria contain some feldspar but consist mostly of pyroxene,
olivine, and ilmenite, which are minerals that are rich in iron and poor in
aluminum.

The
concentration of iron or aluminum serves as a useful chemical classification
system in lunar rocks. Lunar meteorites that are mare basalts (e.g., NWA 032) or breccias composed mainly of mare
material (EET 87521/96008) are poor in
aluminum and rich in iron. In contrast, meteorites from the feldspathic
highlands are rich in aluminum and poor in iron. Glass spherules and basalt
fragments from the maria have been found as clasts in most of the highlands
meteorites and some (e.g., Yamato 791197)
contain a higher proportion of mare material than others. Such meteorites
plot on the high-iron end of the range of highlands (feldspathic) lunar
meteorites. Some intermediate lunar meteorites (e.g., QUE 94281) apparently derive from a place
where the mare and the highlands are in close proximity because they are
breccias consisting of clasts of both mare and highlands rocks. (All regolith
samples from the Apollo 15 and 17 missions are mixed in this way.) Such
meteorites have intermediate concentrations of iron and aluminum. We might
expect, as more lunar meteorites are found, that the gaps in the
aluminum-iron plot above will be filled in.

Why Are Lunar Meteorites Important?

It
may seem, considering that 382 kg of
well-documented rock and soil samples were obtained from nine locations
by the Apollo and Luna missions, that a few small rocks from unknown points
on the lunar surface cannot be very important. For several reasons, however,
the lunar meteorites have provided new and useful information.

The
Apollo missions all landed in a small area on the lunar nearside, and some of
those missions were deliberately sent to sites known to be geologically
"interesting," but atypical of the Moon. (On Earth, Yellowstone
National Park is geologically "interesting," but hardly typical.)
The gamma-ray
and neutron spectrometers on the Lunar
Prospector mission (1998-1999) have shown that all of the Apollo sites
were in or near a unique and anomalously radioactive "hot spot" on
the lunar nearside in the vicinity of Mare Imbrium. This existence of this
hot spot, sometimes known as the Procellarum KREEP Terrane or PKT, indicates
that the mare-highlands distinction of the ancient astronomers is not
adequate in a geochemical sense. Many rocks collected on the Apollo missions
that likely originated from the PKT (especially those from Apollos 12, 14,
and 15) are neither mare basalts nor feldspathic breccias. They are rocks
(usually impact-melt breccias) of intermediate FeO concentration (~10%) with
high concentrations of the naturally occurring radioactive elements: K
(potassium), Th (thorium), and U (uranium). Such rocks are often called
"KREEP" because, in addition to K, they have high concentrations of
other elements that geochemists call incompatible
elements such as the rare-earth elements (REE, like lanthanum and
cerium) and phosphorus (P). Lunar meteorite Sayh
al Uhaymir 169 with a whopping 30 ppm Th is a "KREEPy"
meteorite. Almost certainly, it derives from the PKT. Other meteorites that
have high concentrations of Th, like NWA
4472/4485 and Dhofar 1442 also likely
originated in or near the PKT. Most of the rest of the lunar meteorites
appear to have come from outside the PKT because they have low concentrations,
typically <1 ppm, of Th. This distribution is reasonable in that we
believe that the lunar meteorites are rocks from randomly distributed
locations on the lunar surface, and most locations on the lunar surface are
not high in radioactivity.

The map on the top part of the diagram shows
the distribution of the concentration of thorium (Th, in parts per
million), a naturally occurring radioactive element, on the lunar surface
as determined by the gamma-ray spectrometer on Lunar Prospector,
which orbited the Moon in 1998 and 1999 (Lawrence
et al., 2000 and Gillis et
al., 2004). The center of the map shows the nearside and the
left and right edges show the far side of the Moon. The locations of the
six Apollo (A) and three Russian Luna (L) landing sites are indicated (all
on the nearside). The bottom part of the diagram shows the concentrations
of Th in lunar meteorite source craters. (This means, for example, that the
LAP meteorite, NWA 4734, and NWA 032/479 count as 1 source crater because
all 3 meteorites likely came from a single crater.) Most lunar meteorites
have low Th concentrations but a few have high concentrations (see last
column of the List).
The figure shows that (1) the Apollo missions all landed in or near a
region of the Moon with anomalously high radioactivity (the anomaly, which
we call the PKT (Procellarum KREEP Terrane) was not known at the time of
Apollo site selection) and (2) most of the lunar meteorites must come from
areas of the Moon that are distant from the PKT because most have low Th.
Thus, one of the values of the lunar meteorites is that they are samples
from places on the Moon that are more typical of the lunar surface (low
radioactivity) than the Apollo samples. The histogram on the bottom assumes
that the known lunar meteorites derive from 39 source craters. The
impact-melt breccia of SaU 169 plots off scale at 30 ppm; the bar at 9.8
ppm Th represents the regolith-breccia lithology. The figure is an updated
(July, 2007) version of Figure 5 of Korotev
et al. (2003).

Also,
most of the lunar meteorites are breccias composed of fine material from near
the surface of the Moon. This fine-grained material has been mixed by many
impacts. As a consequence, the composition and mineralogy of a brecciated
lunar meteorite is likely to be more representative of the region from which
it came than any single unbrecciated (igneous) rock from the same
region.

We
know that over much of the Moon, and most of the far side, the material of
the lunar surface has only 3-6% FeO because it is highly feldspathic.

Map of the surface concentration of
iron (expressed as FeO) on the lunar nearside (left) and far side (right),
based on spectral reflectance measurements taken by the Clementine
mission in 1994. The FeO data, from 70°S to 70°N, overlays a shaded relief
map. High-FeO areas occur where volcanic lavas (mare basalts) filled giant
impact craters. Low-FeO areas correspond to the feldspathic highlands.
Image courtesy of Jeff Gillis.

About
half of the lunar meteorites have 3-6% FeO, thus these meteorites are
entirely consistent with derivation from typical feldspathic highlands:

These diagrams compare the distribution
of the concentration of iron, expressed as % FeO, in the lunar meteorites
(top) with the lunar surface as measured with the gamma-ray spectrometer on
Lunar
Prospector (middle) and estimated from spectral reflectance
measurements taken by the Clementine
(bottom). Because the distributions have the same shape and because the
peak occurs at the same concentration, we can reasonably infer that the
lunar meteorites are random samples from the surface of the Moon. The large
peak at ~5% FeO corresponds to far side highlands and the small peak at
~19% FeO corresponds to nearside maria (see map). The lunar meteorite data
are updated (end of 2014) from Korotev
et al (2003). Clementine data are from Lucey et al. (2000) and Gillis et al. (2004). The Lunar
Prospector data are from Prettyman
et al. (2006).

These
various factors lead to the ironic circumstance that the feldspathic lunar
meteorites together provide us with a better estimate of the composition and
mineralogy of the typical highlands surface than we were able to obtain from
the Apollo samples.

The
lunar meteorites have also provided us with crystalline mare basalts that are
different from any collected on the Apollo and Russian Luna missions. In
particular, the Northwest Africa 773 stones
are different from any rock in the Apollo collection (e.g., Jolliff et al., 2003).